Fresh air will come in the center vents when the flaps are open, but when the A/C is running, the air will be conditioned. It would be counterproductive to have conditioned air blowing out the side and ambient outside air blowing through the center vents.

When in heating mode, the center vents will tend to close with most of the air going through the side vents and partially through the defrost nozzle.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]